With the maturity of automatic sheet feeding technologies, automatic document feeders are widely used in a diversity of document processing machines such as scanners, printers, and multifunction peripherals. An automatic document feeder is used for successively feeding a stack of sheet-like documents into the document processing machine so as to implement associated operations in a labor-saving manner. Generally, the document processing machine is usually equipped with a sheet size detecting mechanism to be collaboratively used with the automatic document feeder. For smoothly processing the document, before the document is fed into the document processing machine by the automatic document feeder, the size of the document is detected by the sheet size detecting mechanism.
FIG. 1 is a schematic perspective view illustrating a conventional sheet size detecting mechanism disclosed in Taiwanese Patent No. 00588545. FIG. 2 is a schematic top view illustrating a logic plate and a sensing module of the sheet size detecting mechanism as shown in FIG. 1. As shown in FIGS. 1 and 2, the sheet size detecting mechanism 1 comprises a sheet input tray 11, a logic plate 12, and a sensing module 13. The sheet input tray 11 comprises a sheet holder 111 and two opposed sheet guides 112 and 113. The relative positions between the sheet guides 112 and 113 are adjustable. That is, the sheet guides 112 and 113 are movable toward each other or away from each other in order to clamp both sides of the paper sheet. The sensing module 13 is fixed on a bottom surface of the sheet input tray 11. The logic plate 12 is connected with the sheet guide 113. Consequently, the logic plate 12 and the sheet guide 113 may be synchronously moved. The logic plate 12 is a strip-shaped plate. In addition, plural featured structures 121 are formed on a surface of the logic plate 12. By moving the sheet guide 113 to comply with the width of the paper sheet, a relative motion between the logic plate 12 and the sensing module 13 is caused and plural digital signals are generated. According to the digital signals, the sheet size can be judged. This sheet size detecting mechanism, however, still has some drawbacks. For example, the strip-shaped logic plate 12 is costly. In addition, the use of the logic plate 12 occupies much space of the sheet input tray 11.
For solving the above drawbacks, a sheet size detecting mechanism for saving space of the sheet input tray is disclosed in for example U.S. Pat. No. 6,070,048. FIG. 3 is a schematic perspective view illustrating a sheet size detecting mechanism disclosed in U.S. Pat. No. 6,070,048. As shown in FIG. 3, the sheet size detecting mechanism 4 comprises a sheet guide 41 with a rack part 411, a circular gear 42, and a variable resistor 43. The rack part 411 of the sheet guide 41 is engaged with the circular gear 42. The circular gear 42 is connected with the variable resistor 43. Consequently, the circular gear 42 and the variable resistor 43 are synchronously rotated. When the sheet guide 41 is moved to comply with the width of the paper sheet, the circular gear 42 is correspondingly rotated and thus the variable resistor 43 generates an output signal. The resistance value of the output signal is varied according to the rotating amount of the circular gear 42. That is, the sheet size may be detected according to the resistance value of the output signal. Since the variable resistor 43 is not cost-effective, the sheet size detecting mechanism is still unsatisfied.